1. Field of the Invention
The present invention relates to a video signal processing circuit and a video signal processing method for converting the number of scan lines so that the video signal achieved by subjecting a video signal of an NTSC system to double data rate conversion is converted to a video signal of an HDTV (High Definition TV) system, and an image display device using the same.
2. Description of the Related Art
The NTSC (National Television System Committee) system in which the aspect ration is set to 4:3 and the number of vertical scan lines is equal to 525 (the number of effective scan lines containing video signals is equal to 480) has been known as a television system. Recently, the high definition (EDTV-II: Extended Definition Television) system in which the aspect ratio is set to 16:9 has been also known. In addition, the HDTV (High Definition Television) system in which the aspect ratio is set to 16:9 and the number of vertical scan lines is equal to 1125 (the number of the effective scan lines is equal to 1080) has been known as a television system.
For example, when a video signal of the EDTV-II system is subjected to double data rate conversion by using a line doubler, the number of vertical scan lines of the video signal after the conversion (hereinafter referred to as “double-rate signal”) is equal to 1050 (the number of the effective scan lines thereof is equal to 960).
This double-rate signal is different in horizontal frequency from the video signal of the HDTV system (hereinafter referred to as “HDTV signal”). Therefore, when an image based on a double-rate signal is displayed on a display device such as a cathode ray tube which is scanned at the horizontal frequency corresponding to the HDTV signal, the number of scan lines of the double-rate signal is converted to nine eighth ( 9/8 time) so that the horizontal frequency of the double-rate signal approaches to that of the HDTV system.
Therefore, the horizontal frequency of the double-rate signal is equal to 31.5 kHz(=30 Hz×1050) because the frame frequency is equal to 30 Hz and the number of vertical scan lines is equal to 1050. On the other hand, the frame frequency of the HDTV signal is equal to 30 Hz and the number of vertical scan lines of the HDTV signal is equal to 1125, so that the horizontal frequency of the HDTV signal is equal to 33.75 kHz(−30 Hz×1125). The conversion of the number of scan lines (herein after referred to as “scan line number conversion”) is generally carried out on the basis of a linear interpolation calculation although the interpolation is carried out by using an interpolating filter which approximates to the interpolation function of sin(x)/x in some cases when much attention is paid to the image quality.
FIG. 13 shows the construction of a scan line interpolating circuit 200 for performing such a linear interpolation calculation.
The scan line interpolating circuit 200 has an input terminal 201 to which a double-rate signal VDU is input, and a delay circuit 202 for delaying the double-rate scan line signal VDU input to the input terminal 201 by only one horizontal period (1H). Here, it is assumed that the double-rate signal VDU input to the input terminal 201 has been read out, for example from a buffer memory (not shown), on the basis of signals achieved by multiplying the vertical, horizontal synchronous pulses of the HDTV signals.
The scan line interpolating circuit 200 further has multipliers 203a to 203h for multiplying the double-rate signal VDU input to the input terminal 201 by coefficients of 0.89, 0.78, 0.67, 0.56, 0.44, 0.33, 0.22 and 0.11 respectively, multipliers 204a to 204h for multiplying the double-rate signal VDU delayed in the delay circuit 202 by only one horizontal period by coefficients of 0.11 0.22, 0.33, 0.44, 0.56, 0.67, 0.78 and 0.89 respectively, and adders 205a to 205h for adding the output signals of the multipliers 203a to 203h with the output signals of the multipliers 204a to 204h respectively.
Further, the scan line interpolating circuit 200 has a nonary counter 206 for repetitively counting the numerals from “0” to “8”, a change-over switch 207 to which the count output of the counter 206 is supplied as a switching control signal, and an output terminal 208 for outputting a signal achieved at the movable terminal of the change-over switch 207 as an output video signal VDU′.
The fixed terminal at the side a (hereinafter referred to as “fixed terminal a”) of the change-over switch 207 is connected to the input terminal 201, and the fixed terminals of the sides b to i (hereinafter referred to as “fixed terminals b to i” respectively) are connected to the output sides of the adders 205a to 205h respectively.
A horizontal synchronous pulse Ph of the HDTV signal is supplied as a count clock to the counter 206. The movable terminal of the change-over switch 207 is connected to each of the fixed terminals a to i respectively when the count output of the counter 206 varies from “0” to “8” respectively.
Next, the operation of the scan line interpolating circuit 200 will be described.
During the horizontal period for which the count output of the counter 206 is equal to “0”, the movable terminal of the change-over switch 207 is connected to the fixed terminal a, a signal L1 of the first scan line of the double-rate signal VDU is input to the input terminal 201 and the signal L1 of the first scan line is directly passed through the side a of the change-over switch 207 and output as a signal L1′ of the first scan line of the output video signal VDU′ to the output terminal 208.
During the horizontal period for which the count output of the counter 206 is equal to “1”, the movable terminal of the change-over switch 207 is connected to the fixed terminal b, a signal L2 of the second scan line of the double-rate signal VDU is input to the input terminal 201 and the signal L2 of the second scan line is multiplied by 0.89 in the multiplier 203a. At the same time, the signal L1 of the first scan line of the double-rate signal VDU is output from the delay circuit 202, and multiplied by 0.11 in the multiplier 204a. Thereafter, the output signals of the multipliers 203a and 204a are added with each other in the adder 205a, and the addition signal (0.89L2+0.11L1) is passed through the side b of the change-over switch 207, and output as a signal L2′ of the second scan line of the output video signal VDU′ to the output terminal 208.
The same operation is carried out when the count output of the counter 206 is equal to “2” to “8”, whereby the signals of nine scan lines of the output video signal VDU′ are created from the signals of eight scan lines of the double-rate signal VDU.
FIG. 14 shows the interpolation of the scan lines in the respective cases where the count output is equal to “0” to “8” as described above.
When the conversion of the scan line number is performed by the linear interpolation as described above, degradation of image quality occurs. The degradation of the image quality will be described with reference to FIG. 15.
FIG. 15 shows the state of the signal level of each scan line of the output video signal VDU′ in the case where the signal level of the scan lines of the double-rate signal VDU alternately repeats each of the white level and the black level by every two scan lines.
In this case, a white portion corresponding to two scan lines of the double-rate signal VDU is expanded to a portion corresponding to three or four scan lines in the output video signal VDU′. In the double-rate signal VDU, the two scan lines in the white portion have the same brightness. However, in the output video signal VDU′, two bright scan lines are different in brightness from each other, or only one scan line is bright.
The degradation of the scan line image quality as described above is not caused by the conversion manner, and it can be prevented by limiting the band of the input signal. However, if the scan line signal is subjected to the band limiting, an image would be blurred, so that the band limiting cannot be actually adopted. Therefore, when the conversion of the scan line number is carried out by the linear interpolation, the degradation of the image quality is unavoidable.